Description
Summary Abstract
Enclomiphene (trans-clomiphene citrate; (E)-clomiphene; Androxal; CAS 7599-79-3) is the trans-geometric isomer of clomiphene citrate and a nonsteroidal selective estrogen receptor modulator (SERM) with predominant estrogen receptor antagonist activity. By blocking hypothalamic estrogen receptors, it interrupts negative feedback and drives pulsatile gonadotropin-releasing hormone (GnRH) secretion, elevating both luteinizing hormone (LH) and follicle-stimulating hormone (FSH). The resulting gonadotropin surge stimulates endogenous testicular testosterone synthesis without directly suppressing spermatogenesis — a clinically significant distinction from exogenous testosterone replacement therapy (TRT). Phase 2 and Phase 3 trial data demonstrate significant and sustained testosterone normalization in hypogonadal men, with concurrent preservation of semen parameters that decline under TRT.
Clinical Indications
Enclomiphene has been evaluated in multiple Phase 2 and Phase 3 clinical programs for male endocrine research:
- Secondary Hypogonadism: Restores serum testosterone to eugonadal range in men with hypothalamic-pituitary suppression, as demonstrated in randomized trials versus placebo and versus topical testosterone gel, with statistically significant T elevations maintained over 3–6 month treatment periods.
- Fertility Preservation: Unlike exogenous TRT, enclomiphene maintains LH and FSH drive to the testes, preserving sperm count and motility in studies directly comparing it to topical testosterone; multiple published trials confirm superiority of enclomiphene over TRT for semen parameter preservation.
- Metabolic and Body Composition Research: Secondary endpoints across trials indicate improvements in lean body mass and energy consistent with testosterone normalization; investigated in the context of obesity-related hypogonadism.
Contraindications
- Primary Hypogonadism (Testicular Failure): Enclomiphene requires an intact hypothalamic-pituitary-gonadal (HPG) axis; it is ineffective in primary hypogonadism where the testes lack functional Leydig cells.
- Estrogen-Sensitive Tissue Models: As a SERM, enclomiphene may exert tissue-selective agonistic or antagonistic effects depending on ER subtype and co-regulator availability; co-incubation studies with ER-positive cells require full estrogenic pharmacology controls.
- Zuclomiphene Contamination: Racemic clomiphene citrate contains approximately 38% zuclomiphene (the cis-isomer), which has estrogenic agonist properties and a markedly longer half-life; studies using enclomiphene specifically should confirm isomeric purity.
Mechanism of Action (MOA)
Enclomiphene restores gonadotropin signaling through competitive estrogen receptor antagonism at the hypothalamic-pituitary axis:
Hypothalamic ER Antagonism
Enclomiphene binds hypothalamic estrogen receptors with high affinity, blocking the negative feedback exerted by circulating estradiol on GnRH pulse frequency and amplitude. The resulting disinhibition increases GnRH secretion, driving upstream gonadotropin release from the anterior pituitary.
LH and FSH Elevation
Elevated GnRH stimulates the anterior pituitary to increase secretion of both luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH acts on testicular Leydig cells to stimulate steroidogenesis; FSH acts on Sertoli cells to maintain spermatogenesis — an axis that TRT suppresses via exogenous androgen feedback.
Endogenous Testosterone Synthesis
The LH surge drives intratesticular testosterone production from Leydig cells. This maintains the high intratesticular testosterone concentration required for normal spermatogenesis (3–10× higher than serum levels). Clinical trials confirm serum testosterone normalization at doses of 12.5–25 mg/day with intact sperm parameters.
SERM Tissue Selectivity
As with other SERMs, enclomiphene’s ER binding induces conformational changes in the ligand-binding domain that dictate recruitment of co-activators versus co-repressors in a tissue-specific fashion. This produces antagonism in hypothalamic and pituitary ER contexts while potentially exerting partial agonism in other tissues — an important variable in experimental designs using ER-expressing cell lines.
Key Features & Specifications
Enclomiphene is available in three variant forms covering a full range of oral and liquid dose research formats:
Chemical Analysis
| Property | Specification Reference Data |
|---|---|
| IUPAC Name | (E)-2-[4-(2-chloro-1,2-diphenylethenyl)phenoxy]-N,N-diethylethanamine, 2-hydroxy-1,2,3-propanetricarboxylate |
| CAS Number | 7599-79-3 (citrate salt) |
| Molecular Formula | C26H28ClNO · C6H8O7 (C32H36ClNO8) |
| Molecular Weight | 598.1 g/mol (citrate) |
| Synonyms | Trans-clomiphene citrate; (E)-clomiphene; Androxal; RMI-16289; ICI-46476; enclomid |
| Form / Variation | 6.25 MG Capsules (C-045, $54.00) / 12.5 MG Capsules (C-012, $78.00) / 20 MG/mL Liquid 30 mL (L-001, $68.00) |
Storage, Safety, and Handling
Storage Protocol
Store capsules and liquid formulation at −20 °C, protected from light and moisture. The citrate salt is stable for ≥2 years under proper conditions. Liquid solutions should be sealed tightly after use; confirm clarity and absence of precipitation before each administration in in vivo studies.
Handling & Compliance
Enclomiphene is not FDA-approved; the IND (Androxal) was evaluated through Phase 3 trials. It is not currently a scheduled substance in the US. As an ER-modulating compound, it is listed as a prohibited substance under WADA regulations (S4 — Hormone and Metabolic Modulators). Compliance review is required for all in vivo protocols.